part 2] THE METAMORPHOSED ROCKS OP THE START AREA. 197 



rocks. The conversion of pyroxene to chlorite, and of calcic 

 plagioclase to epiclote, zoisite, and albite, is the normal result of 

 these conditions. Among these rocks there is seldom reason to 

 believe that chlorite is secondarily developed from hornblende. 



The development of chlorite at the expense of pyroxene may 

 have been early initiated, as a weathering reaction in the original 

 lavas and tuft's before their metamorphism. In none of these 

 rocks can any evidence of original (but now relict) minerals be 

 observed, nor are there preserved original igneous textures. In 

 the complete recrystallization, following the chemical changes, 

 original minerals and textures have been completely obliterated. 

 A pseudoporphyritic texture is not infrequently observed ; but 

 there is every reason to believe that this is a porphyroblastic 

 texture developed during metamorphism. The best examples of 

 this are the porphyroblastic albites in the green schists already 

 described. Original ilmenite or titanomagnetite in the igneous 

 rocks gives rise to titanite in the metamorphosed types. The 

 production of chlorite from pyroxene, particularly augite, has set 

 free abundant lime, which has appeared both as epidote and 

 calcite where carbon dioxide was present. Calcite is a common 

 constituent of these schists. In metamorphism under directed 

 pressure, it has often crystallized in narrow streams among the 

 remaining minerals. Its subsequent removal by weathering gives 

 (as already noticed) a pitted and cavernous appearance to some of 

 the rocks, besides accentuating their schistosity. 



Only the larger features have been preserved, such as the 

 nodular masses of epidote, chlorite, calcite, and albite, which are 

 here interpreted as metamorphosed amygdales. 



In the next stage of metamorphism, hornblende largely takes 

 the place of chlorite. Already in the chlorite-epidote-albite- 

 schists, hornblende begins to appear as fine acicular needles 

 associated with chlorite or developed poikiloblastically within 

 albite. With increasing metamorphism, hornblende increases in 

 amount, and the individual crystals themselves become larger : 

 ultimately they may appear as porphyroblasts. Kocks of this 

 type are distributed throughout the area where the Green Schists 

 are exposed, and it is impossible to present a map showing zonal 

 distribution of either type. 



The development of hornblende, of an aluminous type (as shown 

 by the analysis), is the result doubtless of a chemical interaction 

 between chlorite and calcite, or between chlorite and epidote. In 

 both these reactions it is clear that additional silica is required, 

 and is provided by quartz. Small amounts of quartz can usually be 

 detected interspersed among the albite-grains of the Green Schists. 



Not infrequently have I observed that the development of 

 abundant amphibole is accompanied by a noticeable increase in 

 the lime content of the plagioclase, oligoclase, or oligoclase-albite, 

 appearing with or without albite. This is in accord with the 

 higher grade of metamorphism, and the increasing stability of the 

 more basic felspar as metamorphism progresses, 



Q. J. G. S. No. 314. p 



